In general, in a gas turbine that burns a low-calorie blast furnace gas (hereinafter, blast furnace gas turbine), in order to obtain a predetermined turbine output, the low-calorie blast furnace gas large in quantity compared with a normal liquefied natural gas (LNG) is supplied to a combustor as fuel. Therefore, a working fluid amount in the turbine increases compared with gas turbines of a simple cycle (a standard cycle for using kerosene, gas oil, natural gas, or the like as fuel and not performing humidification of an intake air) and a combined cycle. Therefore, when the blast furnace gas turbine is designed and manufactured from the gas turbine of the simple cycle or the combined cycle already designed, a working fluid amount in a compressor is reduced to maintain a balance among a turbine output, and a generator output and compressor power.
However, when the working fluid amount in the compressor is reduced, a pressure ratio of the compressor decreases. Therefore, for example, it is necessary to take some measure for increasing the pressure ratio of the compressor as disclosed in Patent Literature 1.